Spontaneous emulsification of detergent solubilized reaction center: protein conformational changes precede droplet growth

Abstract

We show that ionization of a pH-sensitive detergent, DDAO, bound to a bacterial photosynthetic reaction center (RC), induces reversible emulsification of the protein over a narrow acidic pH range, resulting in stable micrometric RC-surfactant droplets. Electrostatic interactions play a key role in the phase separation process, as shown by a systematic analysis of ionic strength effects and by the use of a cationic detergent (DTAB) that mimics, also at basic pH, the ionized form of DDAO. Under all the conditions we tested, phase segregation seems to be coupled to a 15 nm blue-shift of the low energy absorption band of the primary electron donor P of the RC. This spectral change strongly suggests that surfactant-protein interactions leading to phase separation also induce a conformational transition of the RC. Time-resolved visible-NIR spectra recorded during the emulsification process reveal that the conformational change probed by P spectral shift is always faster than droplets formation. In line with these observations, phase segregation affects charge recombination kinetics following RC photoexcitation, as well as electron transfer from soluble cytochrome c₂ to the photoxidized primary donor P⁺.